A lot of people assumed (I among them) that the famous glitch Pokemon known as Missingno would be removed from the Virtual Console versions of Pokemon Red and Blue (it does not occur in Yellow). Good news — it wasn’t! You can still do it!
To refresh your memory, you first need the ability to Fly and Surf, then you need to talk to an old man in Viridian City to make him demonstrate how to catch a Pokemon. Once he’s finished, Fly to Cinnabar Island and Surf along the little strip in between the land and the sea until you encounter Missingno. Catching the jumbled bar in a Pokeball will create endless duplicates of whatever you have as the sixth item in your inventory, which holds obvious advantages.
The question is, then, how dangerous is Missingno? Back in the day, Nintendo warned of entire saves being corrupted and every Pokemon you had being erased, among other things. Now that what causes Missingno to happen has been studied inside and out, we know having the glitch Pokemon in your inventory usually doesn’t result in a game-ending crash. It’s possible to encounter other glitch Pokemon via the same method, and it’s those that are far more dangerous. Letting Missingno free ends the glitch altogether, until you catch it again.
Pokemon Red, Blue and Yellow are available on the 3DS Virtual Console as of today, Pokemon Day (February 27, the 20th anniversary).